1. Field of the Invention
The present invention relates to an extruder head for producing flat cohesive profiled webs from resilient mixtures of differing composition, such as mixtures of rubber or synthetic material, and especially for producing uncured tread strips for pneumatic tires having a two ply central portion and two similar edge portions. The extruder head includes two extruders which are disposed at an acute angle next to or one above the other, and which feed a common extrusion nozzle via a preform die or member having flow channels which extend one above the other. The extruder head also includes a third extruder which is connected to the preform die at a greater angle than are the previously mentioned extruders; this third extruder conveys the mixture for the edge portions.
2. Description of the Prior Art
It is known, and for a long time has also been the customary practice, to cohesively produce multiply profiled webs by uniting the partial flows conveyed from several individual extruders in a common nozzle. This posssibility is advantageously utilized for preforming or preliminary shaping of uncured tread strips for pneumatic vehicle tires. Since resistance to wear is predominately required for the tread surface region, while flexibility and crack resistance are primarily required for the side portions, the tread strips must basically comprise two different elastomer mixtures. The joining of the partial profiled webs in the extruder head, which unites the two extruders, poses considerable problems, which in practice have in general been overcome with known devices (see, for example, German Pat. No. 1 729 599) Kohlepp et al. corresponding to U.S. Pat. No. 3,584,343--Kohlepp et al. dated June 15, 1971 and belonging to the assignee of the present invention. However, the recently generated desire in the tire manufacturing industry for providing the tread strip with relatively narrow edge portions made of a particularly readily bondable mixture involves an additional complication. The great distance between these edge portions proves to be extremely unfavorable with regard to flow dynamics. Despite careful matching of all flow routes to one another using built-in flow retarders, irregularities repeatedly occurred. These irregularities are manifested in the uncured tread strips by poor bonds and by the appearance of delaminations.
An object of the present invention therefore is to make uniform the flow conditions in the common extruder head, to which three extruders are connected, as a prerequisite for the desired flowing together and satisfactory fusing, and hence to make possible the production of tread strips, all of the portions of which have utility.